The present invention relates to a semiconductor light emitting device for use in optical transmission, particularly in IEEE (Institute of Electrical and Electronics Engineers) 1394, and in display or the like.
In recent years, semiconductor light emitting devices have being broadly applied on such fields as optical communication and information display panels. The semiconductor light emitting devices for use in these applications are required to have high luminous efficiency. In particular, it is important for the semiconductor light emitting devices for use in optical communication to have high response speed.
Recently, a POF (Plastic Optical Fiber) has been started to be used in communication in relatively short distance. As a light source of the POF, there has been developed a surface-emitting rapid-response LED (Light Emitting Diode) having an emission wavelength in the vicinity of 650 nm, which is a low loss wavelength range for the POF. The active layer of this semiconductor light emitting device is made from an AlGaInP (Aluminum Gallium Indium Phosphide) based semiconductor material capable of high efficiency light emission, and has structure of quantum well. As a means to improve light extraction efficiency of the semiconductor light emitting device, a DBR (Distributed Bragg Reflector) is introduced as a multilayer reflecting film with high reflectance placed in between the active layer and a GaAs (Gallium Arsenide) substrate.
FIG. 9 is a view showing an emission spectrum of a semiconductor light emitting device having an active layer provided with the DBR and the quantum well structure, in which the distance between the DBR and the quantum well layer, that is, the distance between the upper surface of the DBR and the lower surface of the quantum well active layer is approximately 1 μm. In FIG. 9, a horizontal axis represents a wavelength of light (nm) while a vertical axis represents relative intensity of light (a.u.: arbitrary unit). When the semiconductor light emitting device emits a ray of light, a ray of light reflected by a DBR on the lower side of the active layer and returned back to the active layer is absorbed little by the active layer and radiated from the semiconductor light emitting device. This is because the active layer having the quantum well structure is extremely small in thickness. Here, an emitted ray of light from the active layer is modulated through interference with a reflected ray of light reflected by the DBR. This changes the spectrum of the ray of light. More particularly, as seen from an emission spectrum configuration in FIG. 9, troughs caused by the interference appear at an interval of approximately 30 to 40 nm in wavelength, generating sub peaks in both sides of a main peak having a wavelength of approximately 665 nm. This indicates that a phase difference between the ray of light reflected by the DBR and the ray of light emitted from the active layer is approximately a multiple of 2π.
However, in the prior art semiconductor light emitting device, the emission spectrum configuration is considerably changed with variance in a distance between the upper surface of the DBR and the lower surface of the active layer, causing a change in the peak wavelength. More particularly, in the semiconductor light emitting device having the emission spectrum shown in FIG. 9, a few % increase in the distance between the upper surface of the DBR and the lower surface of the active layer forms an emission spectrum configuration as shown in FIG. 10. Compared with the emission spectrum configuration in FIG. 9, the peak in FIG. 9 is replaced with a trough of the emission spectrum configuration in FIG. 10, and the wavelength of the main peak moves to a short wavelength side by approximately 15 nm to be approximately 650 nm. In other words, a slight change in the distance between the upper surface of the DBR and the lower surface of the active layer transforms a peak of interference to a trough, causing a displaced peak wavelength.
In the case where the semiconductor light emitting device is used as a light source for POF communication, the low loss wavelength range of the POF is as small as around 40 nm, so that the peak wavelength in the emission spectrum of the semiconductor light emitting device is required to be set within the low loss wavelength range without displacement. In other words, the distance between the upper surface of the DBR and the lower surface of the active layer should be set with high accuracy. Accordingly, in the process of manufacturing semiconductor light emitting devices, a clad layer and the like placed in between a DBR and a quantum well active layer requires high-accuracy layer thickness control in particular. This leads to a problem of decrease in a yield of the semiconductor light emitting devices.